DeepZot on Dark Matter: How we won the Mapping Dark Matter challenge

Daniel Margala and David Kirkby (as team DeepZot) placed first in the Mapping Dark Matter challenge. Daniel agreed to answer a few questions for No Free Hunch as part of our series of posts on the best Mapping Dark Matter entries.

What was your background prior to entering Mapping Dark Matter?

I graduated from the University of California, Los Angeles in 2009 with a B.S. in Physics. In the course of my studies at UCLA, I learned Linux system administration and various scripting languages managing a cluster of servers and data archive for an astro-particle research group. I became interested in numerical analysis while investigating the polarity and momentum of muons produced in the atmosphere by incident cosmic rays. Currently, I am a PhD student in the Physics and Astronomy Department at the University of California, Irvine. My advisor (and DeepZot team member), Prof. David Kirkby, and I are using the Baryon Oscillation Spectroscopic Survey (BOSS) to study the distribution of matter in our universe at the largest volumes. My work with BOSS has primarily focused on the operations software at the telescope, specifically, with the interfaces to the BOSS spectrograph, located at Apache Point Observatory in New Mexico.

How did you come to form a team together?

I became interesting in working with David during a conversation that included an avid discussion of programming languages at a department event (where I was lured by the prospect of free food and drink, the perfect bait for graduate students). I began working on a variety of projects with David for about half a year, ranging from cosmology to electrical engineering, before we started working on the related GREAT10 challenge.

What made you decide to enter?

The GREAT10 challenge was a perfect opportunity for me to bring my freshly developed proficiency in numerical analysis to bear. As a student looking to gain experience, this was also a chance to contribute to the forefront of analysis techniques employed by the weak lensing community. The comparatively compact size and similarity between data sets made participating in the MDM challenge very attractive. The ellipticity measurement (galaxy shape) in the MDM competition was a critical step in our GREAT10 analysis, where the goal is to disentangle the shear (due to dark matter) from the intrinsic galaxy shape.

What was your most important insight into the dataset?

The most important insight was that the pixel-level residuals are a powerful tool for finding the best-fit models for galaxy and star images. We were able to assess the quality of various image models and parameters studying distributions of residuals across sets of images. This was essential to our method, which consisted of a pixel-level maximum-likelihood fit to each star and galaxy image.

The images below demonstrate our fit for a single galaxy image. From left to right, we see the original image (zoomed in on the center), our fitted model with the same resolution, and a higher resolution version of the fitted model:

Were you surprised by any of your insights?

We used an artificial neural network to improve the ellipticity measurements from the fitting procedure. The neural network was trained to provide corrections using a non-obvious subset of the of the likelihood output. For example, feeding the centroid position for a galaxy image to the neural network worsened the predicted correction. Without the neural network, our best entry would have ranked 8th.

Which tools did you use?

Our code consists of C++ libraries that we developed to perform the following tasks: generate images by convolving galaxy and point spread function models, access GREAT10 and MDM images, carry-out maximum-likelihood fit of images, do KSB measurement (moment inspired technique commonly used by astrophysicists) of ellipticity, and provide an ellipticity correction via machine learning algorithms.

The code utilizes a fit minimization engine (Minuit) and neural network engine (TMVA) that are both available as part of the open source (LGPL) ROOT data analysis framework (http://root.cern.ch), which is widely used by particle physicists.

What have you taken away from the competition?

I am intrigued by the tight cluster of scores near the top of the leaderboard despite the wide variety of methods applied. This suggests that those methods are making use of the maximum amount of information available in the images in the presence of pixelation and noise.

Winning the MDM competition gave us confidence in our strategy for measuring the shapes of galaxies, which we have put to use in GREAT10.

Most importantly, why 'DeepZot'?

The name DeepZot was formed by merging 'Deep Thought', the name of a fictional computer from Douglas Adams’ Hitchhiker’s Guide to the Galaxy, and 'Zot!', the battle cry of our school’s mascot, Peter the Anteater.

-What functional forms did you use for the galaxy model and point spread function models, and how did you determine these?
-Which neural network implementation in TMVA did you use?
-What parameters for the network did you use?
-How did you select these parameters?

Daniel Margala

thanks!

-What functional forms did you use for the galaxy model and point spread function models, and how did you determine these?
We ended up using an exponential profile for the galaxy model and a moffat distribution for the PSF. We also experimented with add a de Vacouleurs component to the galaxy model. Our software was designed so that it would be easy to swap and combine models. To assess the quality of our models we studied the pixel-level residuals (stacked from thousands of images) between the predicted and observed image.

-Which neural network implementation in TMVA did you use?
The Multi-Layer Perceptron implementation with BFGS training.

-What parameters for the network did you use?
The parameters from the model fits were fed to the network as inputs. We trained the network to "learn" corrections to the fitted ellipticity values.

-How did you select these parameters?
Trial and error for the most part. We had trouble with convergence feeding ALL of the information we had available for each of the images at first. So we started from a smaller subset of the parameters and systematically added parameters and evaluated the results.